The present invention relates to a disc brake and, more particularly to a disc brake comprising a pressure member guided adjustably, in the direction of a brake shoe, in a brake caliper, and a pressing element operatively arranged with radial play in the pressure member such that, in an initial position of the pressure member, the pressing element projects from a front end of the pressure member facing the brake shoe, is supported on the brake shoe and can be pressed back into the pressure member counter to the effect of a return force which, in the event of mutual contact between the pressing element and brake shoe, is lower than a pressing force of the pressure member.
It is known that, during braking operations, disc brakes can generate disturbing whistling or squealing noises with a sound-pressure level of up to 80 dB. Investigations have shown that such noises can be prevented or reduced to such an extent that at most they still contrast slightly with the normal operating noises of the vehicle.
This is true inasmuch as, even at very low pressing forces, the brake shoe loaded by a pressure member, for example a brake piston, is brought to bear against its associated bearing faces of the disc-brake anchor plate guiding the brake caliper, thereby varying the vibration behavior of the brake shoe correspondingly.
To achieve this, German Auslegeschrift No. 2,047,634, provides, for example, in a disc-brake brake piston forming the pressure member, a cylindrical pressing element which projects beyond the brake piston and is supported on the brake shoe and which is arranged in the brake piston so as to be freely movable to a limited extent perpendicularly to the piston axis.
Furthermore, German Offenlegungsschrift No. 2,022,699 discloses a disc brake in which there is provided as a pressing element a plate which consists of metal or sintered metal powder and which either is fastened to the side of the brake shoe facing the brake piston and is in engagement with the brake piston or is fastened to the latter and is in engagement with the brake shoe.
By way of this plate, a lubricant is held between the mutually opposite faces of the brake shoe and brake piston, in order to reduce friction between these parts or, should a braking force take effect, to allow a transverse movement of the brake shoe in relation to the axis of the brake piston.
It is known from German Offenlegungsschrift No. 2,022,700 to provide rolling bodies, in the form of balls or rollers, between the brake shoe and the brake piston in order to achieve a transverse movement of the loaded brake shoe.
Another disc brake is described in German Utility Model No. 1,962,316. Here, the pressing element forms a solid thrust piece made of elastically deformable material, via which the pressure member forming a piston initially transmits the braking force to the brake shoe. At the same time, the diameter of the piston orifice receiving the thrust piece is larger than that of the thrust piece and corresponds to the elastic deformability inherent in the thrust piece. It is thereby guaranteed that, at the commencement of a braking operation, the thrust piece is gradually compressed or pressed into the piston until the piston itself finally comes to bear, with its end face, against the brake shoe. This ensures that, in braking operations, there is initially no direct heat transfer from the brake shoe to the piston and consequently the hydraulic device actuating the piston can be protected from overheating. Furthermore, the elastic material has a damping effect when the brake disc or disc lining is distorted.
However, the braking forces to be transmitted initially require a compressive strength of the thrust piece which restricts its elasticity such that, at the commencement of a braking operation or in the event of low pressing forces, the brake shoe loaded by the thrust piece cannot be taken along by the brake disc to such an extent that it finally comes to bear against corresponding bearing faces of the brake anchor plate for the purpose of noise reduction.
An object of the present invention is to provide a disc brake which, while being of a construction characterized by technical simplicity, ensures that the brake shoe is taken along reliably, even at low pressing forces, to bear against the brake anchor plate and, at the conclusion of a braking operation, that the pressing element interacting with the brake shoe is returned into its initial position in the pressure member.
This object has been achieved, according to the present invention, by configuring the pressing element as a ring which is supported on a metal spring inserted into the pressure member.
The support of the pressing element, forming a radially movable ring, on a metal spring inserted into the pressure member affords the advantage that, even at low pressing forces and therefore immediately when the pressure member is loaded, an effective reduction of whistling and squealing noises is obtained. Furthermore, even when there is only slight friction between the brake shoe and brake disc, the brake shoe, together with the annular pressing element, is taken along relative to the pressure member, with a corresponding deformation of the metal spring, in the circumferential direction of the brake disc, until it butts against the brake anchor plate.
The construction and support of the pressing element of the present invention thus form an arrangement which reacts sensitively when brought to bear on the brake shoe and which makes it possible, in the initial position of the brake shoe, to keep the annular pressing element in contact with the latter with a specific pressing force and, moreover, as a result of the merely annular bearing of the supporting element against the brake shoe, to keep the heating of the pressure member low during braking operations.
For example, in addition to the disc brake described in the aforementioned to German Utility Model No. 1,962,316, with which the invention is used, German Offenlegungsschrift No. 3,916,741 describes a disc brake whose piston-shaped pressure member already has a pressing element which is inserted into its front end and which, in a similar way to the construction according to the present invention, is arranged so as to move radially in this member and, after the release of the disc brake, always returns reliably into its mid-position or initial position again, irrespective of the extent of its transverse movement.
The pressing element forms a pressure plate which is displaceable within the pressure member on a sliding plate which is inserted into the latter and which opposes a slight frictional resistance to the pressure plate. The return of the pressure plate is brought about by a complex return device which has a plunger arranged coaxially in the brake piston and which passes with one endpiece through the sliding plate at a radial distance and is held in the pressure plate. The other plunger end has a flange, on which a prestressed compression spring surrounding the plunger and bearing against a threaded ring seated in the brake piston is supported.
In the initial phase of the transmission of a braking force to the brake shoe by the brake piston, the pressure plate within the brake piston is displaced radially when the brake shoe comes into engagement with the brake disc. This leads to a tilting of the plunger, with the compression spring being prestressed further on one side. The latter constantly endeavors to pivot the plunger back into its initial position again. When the brake is released, the spring return force increased by the tilting of the plunger therefore causes a return movement of the pressure plate into its initial position, in which the plunger resumes its coaxial position in relation to the piston axis. This construction, in addition to being of a complex design, necessitates complicated assembly work. The present invention can be used to improve this type of disc brake as well.